Simply FrameLoaderStateMachine and FrameLoader::transitionToCommitted().

Source/core/loader/DocumentLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer. 
@@ skipping 280 matching lines @@
     stopLoadingSubresources();
     
     m_isStopping = false;
 }
 
 void DocumentLoader::commitIfReady()
 {
     if (!m_committed) {
         m_committed = true;
         frameLoader()->commitProvisionalLoad();
+        m_writer.setMIMEType(m_response.mimeType());
     }
 }
 
 bool DocumentLoader::isLoading() const
 {
     if (document() && document()->hasActiveParser())
         return true;
 
     return isLoadingMainResource() || !m_resourceLoaders.isEmpty();
 }
@@ skipping 363 matching lines @@
         if (SecurityPolicy::allowSubstituteDataAccessToLocal() && m_originalSubstituteDataWasValid) {
             // If this document was loaded with substituteData, then the document can
             // load local resources. See https://bugs.webkit.org/show_bug.cgi?id=16756
             // and https://bugs.webkit.org/show_bug.cgi?id=19760 for further
             // discussion.
             m_frame->document()->securityOrigin()->grantLoadLocalResources();
         }
 
         if (frameLoader()->stateMachine()->creatingInitialEmptyDocument())
             return;
+        if (frameLoader()->stateMachine()->isDisplayingInitialEmptyDocument())
+            frameLoader()->stateMachine()->advanceTo(FrameLoaderStateMachine::CommittedFirstRealLoad);
         
         // The origin is the MHTML file, we need to set the base URL to the document encoded in the MHTML so
         // relative URLs are resolved properly.
         if (m_archive)
             m_frame->document()->setBaseURLOverride(m_archive->mainResource()->url());
 
         // Call receivedFirstData() exactly once per load. We should only reach this point multiple times
         // for multipart loads, and FrameLoader::isReplacing() will be true after the first time.
         if (!isMultipartReplacingLoad())
             frameLoader()->receivedFirstData();
@@ skipping 471 matching lines @@
     commitLoad(resourceData->data(), resourceData->size());
 }
 
 void DocumentLoader::handledOnloadEvents()
 {
     m_wasOnloadHandled = true;
     applicationCacheHost()->stopDeferringEvents();
 }
 
 } // namespace WebCore